Capacity Fading Mechanism in All Solid‐State Lithium Polymer Secondary Batteries Using PEG‐Borate/Aluminate Ester as Plasticizer for Polymer Electrolytes

Abstract

Solid‐state lithium polymer secondary batteries (LPB) are fabricated with a two‐electrode‐type cell construction of Li|solid‐state polymer electrolyte (SPE)|LiFePO4. Plasticizers of poly(ethylene glycol) (PEG)‐borate ester (B‐PEG) or PEG‐aluminate ester (Al‐PEG) are added into lithium‐conducting SPEs in order to enhance their ionic conductivity, and lithium bis‐trifluoromethansulfonimide (LiTFSI) is used as the lithium salt. An improvement of the electrochemical properties is observed upon addition of the plasticizers at an operation temperature of 60 °C. However, a decrease of discharge capacities abruptly follows after tens of stable cycles. To understand the origin of the capacity fading, electrochemical impedance techniques, ex‐situ NMR and scanning electron microscopy (SEM)/energy dispersive X‐ray spectroscopy (EDS) techniques are adopted. Alternating current (AC) impedance measurements indicate that the decrease of capacity retention in the LPB is related to a severe increase of the interfacial resistance between the SPE and cathode. In addition, the bulk resistance of the SPE film is observed to accompany the capacity decay. Ex situ NMR studies combined with AC impedance measurements reveal a decrease of Li salt concentration in the SPE film after cycling. Ex situ SEM/EDS observations show an increase of concentration of anions on the electrode surface after cycling. Accordingly, the anions may decompose on the cathode surface, which leads to a reduction of the cycle life of the LPB. The present study suggests that a choice of Li salt and an increase of transference number is crucial for the realization of lithium polymer batteries.

Dynamic Transport in Li‐Conductive Polymer Electrolytes Plasticized with Poly(ethylene glycol)–Borate/Aluminate Ester

“…[5,6] Recently, we fabricated an all-solid-state LPB with Li j SPE j LiFePO 4 constructions: This LPB showed degradation after tens of stable cycles arising from anion decomposition at the electrode surface at elevated temperature (60 8C). [7] Hence, we infer that the reduced anion mobility (or higher transport number) is attractive, because it suppresses the polarization (or overconcentration) of anions at the electrode j electrolyte interface during steady-state charging (concentration gradients of anions at the interface are reduced by the decreasing relative anion diffusivity at the steady state). Therefore, the addition of plasticizers with Lewis acidity may be important to realize long-life batteries.…”

Fabrication and electrochemical performance of lithium polymer battery using mesoporous silica/polymer hybrid electrolyte

“…The reaction voltages were observed around 3.4 V between charging and discharging, which agrees with previous results for the two-phase coexistence reaction in an olivine LiFePO 4 cathode. 7), 8) The corresponding capacity retention, Coulombic efficiency, and quasi-polarization with cycle numbers are shown in Figs. 5(a)5(c), respectively.…”

“…Our previous results showed that the decrease in the Li concentration at the SPE/LiFePO 4 interface was caused by the decomposition of the TFSI anion. 8) Thus, the film formed on the cathode particles contained products of the TFSI decomposition.…”

“…The capacity degradation was investigated by using ex situ and in situ techniques, such as electrochemical impedance spectroscopy (EIS), thickness monitoring of the LPB cell, and NMR-imaging techniques. 7), 8) The degradation was attributed to the poor mechanical properties of the SPE, which reduced the electrolyte/electrode contact. This causes an increase of local polarization, resulting in the side reaction of Li salt decomposition at the electrode/electrolyte interface.…”

“…These results are consistent with our previous results using PEO-based SPE with a borate ester. 7), 8) The charge transfer reaction at the electrode/SPE interfaces was investigated further by measuring the temperature evolution of the internal resistances, R 2 and R 3 , from 30 to 70°C.…”